proftpd is slow to transfer

I have installed debian using the setup guide from here and all is fine.

I put shorewall on as my firewall and I am sure this is where the problem lies.

Logging into proftpd is slow and the upload transfer rates fluctuates from 3-5k/s, usually stalling along the way.

I opened up port 20 after investigating and the next time i did an upload it zoomed up at 100k/s. I was very happy until I had another go about an hour later and it's back to it's old tricks.

I'm sure the firewall is blocking the ports it decides to use during transfer.

As i was writing this i turned off Passive File Transfers in the client and it's going at near full speed again. I will leave this thread for now, if it's ok in case it goes back to being slow and I need some advice.